Polyomavirus nephropathy (PVN) is a form of nephritis caused by reactivation of the human polyomavirus, BKV, in renal transplant patients. BKV is ubiquitous in most human populations and establishes a lifelong, subclinical, persistent infection of the kidney. In transplant patients and other immunocompromised individuals, viral replication leads to tissue damage and renal dysfunction. The incidence of PVN has risen dramatically in recent years concomitant with the development of more effective immunosuppressive drug regimens aimed towards preventing rejection of the transplant. Presently, there are no effective anti-viral treatments for BKV, and therefore the clinician is faced with the dilemma of reducing the dose of immunosuppressive drugs so as to allow the patient's immune system to battle the virus, which then raises the risk of rejection. The biology of BKV is not well understood, particularly in human kidney epithelial cells. Such an understanding will be critical in the rational design of therapeutic approaches. Using an in vitro cell culture system for the propagation of primary human proximal tubule epithelial (HPTE) cells, which allows them to maintain their differentiated function, the life cycle of BKV can be studied in detail. Preliminary results indicate that BKV replicates efficiently in these cells, thereby reproducing the situation in the immunosuppressed patient. The long-term aims of this project are to utilize this in vitro system to characterize the details of the viral life cycle and to define conditions that will prevent viral replication similar to those the virus would encounter in a healthy host. A detailed investigation of viral gene expression will be performed. In addition, a thorough analysis of the interactions between the virus and the host cell, focusing on the role of chemokines and cytokines in modulating the infection, will be undertaken. The findings from these in vitro studies will be correlated with an examination of biopsies obtained from patients undergoing renal transplantation at the University of Michigan Hospital to confirm that the in vitro observations mimic the events that occur in the patient. The results of the proposed studies will greatly enhance our knowledge of the mechanism by which BKV is reactivated in the absence of a functioning immune system and therefore point the way to better treatment modalities.